The present invention relates to the general field of aviation.
It relates more particularly to regulating the thrust (i.e. the propulsive force) from a gas turbine engine of an aircraft during a stage of flight of the aircraft, such as a takeoff stage, for example.
The invention has a preferred application in a context in which it is proposed during a stage of flight of the aircraft to reduce the thrust from a gas turbine engine relative to a reference “limit” thrust normally used for this stage of flight (e.g. during the takeoff stage, relative to thrust that may be referred to as “full throttle post-combustion” thrust or FTPC thrust).
This reduction in thrust, also known as “thrust derating” presents several advantages.
Thus, in the first place, the noise produced by the engine during this stage of flight is reduced.
Furthermore, reducing thrust serves to limit stresses exerted on the components of the engine, in particular by lowering the temperature of the exhaust gas leaving the hot portions of the engine (after the combustion chamber): the reliability and the lifetime of these components is thus increased.
The fuel consumption of the engine is also decreased.
Thrust derating is a function commonly used in civil aviation during a takeoff stage for engines of the bypass turbojet (or turbofan) type. It leads to a reduction in the setpoints for the engine speed that is delivered to the turbojet by the full authority digital engine control (FADEC) device.
For a bypass turbojet, the thrust setpoint delivered by the engine regulator device during a stage of aircraft takeoff can be modeled as a function of outside temperature, in the manner shown in FIG. 1.
It is in the form of two curve portions P1 and P2 that are practically linear but with different slopes, which portions connect together at a discontinuity or break point CP. The abscissa value of the break point in FIG. 1 is the limit temperature T0.
In compliance with the relationship shown in FIG. 1, the thrust is regulated by the turbojet regulator device so that for a reported outside temperature higher than the limit temperature T0, the regulator system decreases the thrust setpoint (i.e. the speed of the turbojet) in order to limit the temperature of the outlet gas from the turbojet.
Given this behavior of the regulator device, a known mechanism for derating thrust consists in tricking the regulator device of the engine by informing it that the outside temperature is higher than its real value, and in particular higher than the limit temperature T0. As a result, the regulator device prepares a thrust setpoint for the engine on takeoff that is smaller than the “full throttle post-combustion” setpoint, in compliance with the regulation relationship modeled in FIG. 1.
The outside temperature for tricking the regulator device is also known as the “flex” temperature (or Tflex). It is supplied by the pilot to the regulator system and it is determined on the basis of tables that have been pre-established for various flight conditions (type of airplane, runway, airplane load, wind, etc.). The pilot activates (or deactivates) the changeover to operating the engine at reduced thrust merely by acting on a control lever of the airplane, in known manner (e.g. by positioning the lever in a determined position).
The drawback of that mechanism for derating thrust is that it is limited to gas turbine engines presenting a thrust regulation relationship as a function of outside temperature that is similar to the relationship shown in FIG. 1, i.e. that presents a break point beyond which the thrust setpoint prepared by the regulator system is reduced.
It so happens there exist gas turbine engines for which such a model is not appropriate (e.g. because there is no break point), and for which biasing the outside temperature does not suffice in order to be able to reduce thrust in controlled manner, i.e. with a known reduction factor. A particular example of such an engine is a gas turbine engine having an exhaust gas ejection nozzle of variable section.
There therefore exists a need for an alternative mechanism that enables the thrust of a gas turbine engine to be reduced relative to a reference limit thrust and that is capable of adapting to various types of gas turbine engine, which may have one or more degrees of freedom for regulating thrust (e.g. engine speed, nozzle section, reference limit thrust, etc.).